1. Field of the Invention
This invention relates to metallurgical treatments of steels, especially to methods for hardening only selected regions of a steel surface or structure.
2. Description of the Prior Art
One branch of metallury consists of those treatments used to harden steel surfaces. Hardening treatments may be selected from the group consisting of carburizing, nitriding, carbonitriding, siliconizing, chromizing and boronizing, or combinations of these treatments such as carburizing and boronizing.
A combination of carburizing and boronizing methods is disclosed in U.S. Pat. No. 3,922,038, Wear Resistant Boronized Surfaces and Boronizing Methods, Nov. 25, 1975, which is used commercially to harden the friction bearing surfaces of earth boring bits used to drill oil, gas and other mineral producing wells.
The area around each seal of an earth boring bit having a boronized friction bearing surface is generally protected from the boronizing treatment. Protection by masking may be achieved by copper plating or application of one of a variety of lacquers, pastes or clays. Masking is achieved mechanically in the manufacture of other products with a wrought steel plate or a powdered metallurgical composition, U.S. Pat. No. 4,165,243, Method of Making Selectively Carburized Forged Powder Metal Parts, Aug. 21, 1979. Also, masking with a protective covering body consisting of glass ceramics machinable with conventional work tools is known, U.S. Pat. No. 3,873,376, Method For Case Hardening Workpieces, Mar. 25, 1975.
One effective masking method in the manufacture of earth boring bits uses a split steel ring to meter the height of the nonboronized seal area. A masking clay is worked into the area between the friction (journal) bearing surface and a bevel on the ring. Additional clay is used to seat the boronizing cup before being packed with boronizing compound. Even though effective, there are problems with this method. It is time consuming and relatively costly to use the combination of steel ring and clay. When the cup and steel ring are removed after the boronizing furnace treatment, the seal area is sometimes scratched by the edges of the ring. Then it is sometimes necessary to rework or scrap the bearing and its supporting head section.